About John Chan

I’ve been asked to examine slate roof installations all across the U.S., and one of the most disheartening things I’ve observed is how often incorrect side laps are used. For example, the photo shows a slate roof that was installed less than one year ago and already has more than a dozen leaks. Why? Among one of the most basic problems is the side lap.

This slate roof was installed less than one year ago and already has more than a dozen leaks. One of the most basic problems is the side lap. PHOTO: John Chan

The side-lap detail drawing that appears on this page is from the National Slate Association’s Slate Roofs: Design and Installation Manual, page 86, Detail 5-B. The side lap also is referred to as a side joint, vertical joint, keyway, bond line or rain course. As defined in the glossary of the NSA manual, it’s “the longitudinal joint between two slate shingles”.

Whenever one is installing a slate roof, it is absolutely imperative the side lap is a minimum of 3 inches. As seen in the detail, if the lap is less than 3 inches, water will flow in between the two slates and leak into the building. When I’m asked to inspect a problematic new slate roof, I find the side and head laps are the problems on a majority of all cases across the country.

If you’re installing a single-sized slate, such as 20 by 12 inches, the slates should be installed so the joints are exactly split in two; the side laps on the whole roof should be 6 inches. Similarly, if the roof has 10-inch-wide slate, the side laps should be 5 inches. Whatever the width, the side lap should always be one-half the width on a single-sized slate.

It gets a little trickier on a random-width slate roof. Slate widths can be as narrow as 6 inches or as wide as 20 inches or more. When dealing with 6-inch slates, the joint obviously must be split exactly in the center, so there are 3 inches on each side. If you question the width, pull out a tape measure; this will save you and the building owner lots of money and headaches. When there are inadequate side laps, inevitably, the owner, architect or general contractor gets concerned, and then I get a phone call to do a full roof survey on the slate roof.

Click to download a larger version of this side-lap detail drawing from the National Slate Association’s SlateRoofs: Design and Installation Manual, page 86, Detail 5-B.

As slaters become more advanced, they are able to eyeball 3 inches extremely well, but until that point, installers should use a tape measure, or they should stick with using single-sized slates. It might seem too easy, but this is one of the most common errors I encounter. If a slater studies Detail 5-B and adheres to it, he or she will avoid having this problem with slate roof installations.

Too often, slate is given a bad name because of poor installation. Hopefully, this article and detail will resolve that problem.

Slate has long played a part in the architecture of the new world. It was first introduced to the U.S. as ballast for ships coming across the Atlantic in the 1600s. Slate roofs have been found by archeological excavations in Jamestown, Va., dating as early as 1625, according to Preservation Brief 29, “The Repair, Replacement, and Maintenance of Historic Slate Roofs,” from the National Park Service, Washington, D.C.

Nearly a decade after our nation’s independence, the first slate quarry opened in Peach Bottom Township, Pa. From 1785, slate roofing production grew rapidly and hit its all-time peak around 1914. There were active quarries in Maine, New York, Pennsylvania, Vermont and Virginia. In 1915, slate roofing began to slowly decline because WWI took many of the able-bodied men, leaving quarries short of workers.

In 1922, the Poultney, Vt.-based National Slate Association was formed to promote the use of slate in roofing and structural applications. Many of the first U.S. slate standards were set in a book by the organization called Slate Roofs. However, the association became inactive in the late 1920s because of a lack of cohesion.

In 2002, a historic meeting took place in Sarasota Springs, N.Y. A group of slate manufacturers, roofers, architects, consultants and other interested parties met and reorganized the National Slate Association and formed a three-member board of directors. Matt Millen, Millen Roofing, Milwaukee, became the first president of the association. Today, the board of directors
includes 15 diverse members who are determined to ensure slate will be used extensively around the country. The National Slate Association’s purpose is to promote excellence in slate roofing practices through the development and dissemination of technical information, standards and educational resources.

To ensure it meets its goals, the association has implemented hail testing for the purpose of knowing the thicknesses of slates needed in hail-prone areas and fire testing to certify slate as a fire-resistant material for insurance companies. It also released Slate Roofs: Design and Installation Manual, which won the Rosemont, Ill.-based National Roofing Contractors Association’s Gold Circle Award for outstanding service to the industry. (See “A Review of Slate Roofs: Design and Installation Manual”, to learn more about the manual’s contents.)

The National Slate Association’s current board of directors includes President John Chan, The Durable Slate Co., New Orleans; Senior Vice President Robert Fulmer, Fulmer Consultants, Freeport, Maine; Vice President Dave Large, North Country Slate, Toronto; Vice President Brian Chalsma, The Roofing Co., Hampton, Va.; and Treasurer Pete Papay, Penn Big Bed Slate Co., Slatington, Pa. Jeff Levine, Levine and Co., Ardmore, Pa., is the immediate past president, and Julie Palmer, also of Levine and Co., is the office manager.

Although slate had been used as ballast for ships crossing the Atlantic as early as the mid-1600s, its use was somewhat sparse in the Carolinas until after the great fire. With the Civil War in full bloom, a catastrophic fire broke out in Charleston in 1861, and the city was decimated. However, the Great Reconstruction Era (1865-77) brought shiploads of slate and bricks from North Wales. Welsh slate from the Penrhyn quarries and bricks and tiles from Flintshire and Chester made their way to nearby Liverpool, England, and ultimately to the historic Battery of Charleston.

Not to be outdone, the American quarries started to ship to the Carolinas also. New quarries opened up all along the New York and Vermont corridor, and, in the South, the Virginia Buckingham Co. started quarrying slate in 1867. Slate roofing was growing exponentially at this time, and the Carolinas were consuming it at a very rapid rate.

This dormer features Vermont Black installed in a German style.

As a large port city, Charleston was able to acquire a wealth of different types of slate for its roofs: purple and gray slates from Penrhyn, Wales; Pennsylvania black slates; lustrous black Buckingham slates from Virginia; and greens, purples and reds from Vermont. Although it took more than a decade, Charleston was rebuilt in a grand manner with beautiful slate roofs as far as the eye could see.

Unfortunately, in 1989 Hurricane Hugo struck Charleston, causing nearly $6 billion in damage. The silver lining was many of these historic properties with slate roofs were 100 to 200 years old by 1989 and were in need of major restoration. From 1989-91, Charleston experienced a huge building boom with the insurance companies footing the bill for the restoration of the city. Tradespeople skilled in historic restoration were called in from all over the country and world. Among them were slate roofers hired to assess and restore the city’s slate roofs.

Learning Experience

Having only been a slate roofer for four years at the time, Charleston proved to be a great learning experience for me. Often working 12- to 15-hour days to keep up with the workload, I was able to personally observe various slating techniques from more than a century ago.

For example, still one of the most unique slate roofs I’ve encountered in my 20-plus years in slate roofing, was on a private residence on King Street. It had sustained minimal damage, and in the process of our repairs, we could see why. The entire slate roof was laid in a bed of mortar with wooden pegs where one usually finds nails. Needless to say, it was quite an adventure to restore it back to its prominence.

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May/June 2020

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